From deb23997c532b30392f09bd6d763aacc3c6e6b91 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Julien=20Valverd=C3=A9?= <julien.valverde@mailo.com>
Date: Thu, 16 Jul 2026 23:51:29 +0200
Subject: [PATCH] Add mapStream

---
 packages/effect-lens-next/src/View.ts    | 19 +++++++++++++++++++
 packages/effect-lens/src/Subscribable.ts | 19 +++++++++++++++++++
 2 files changed, 38 insertions(+)

diff --git a/packages/effect-lens-next/src/View.ts b/packages/effect-lens-next/src/View.ts
index bc0160d..f8a6f38 100644
--- a/packages/effect-lens-next/src/View.ts
+++ b/packages/effect-lens-next/src/View.ts
@@ -99,6 +99,25 @@ export const mapOptionEffect: {
     onNone: () => Effect.succeed(Option.none()),
 })))
 
+/**
+ * Allows transforming only the `changes` stream of a `View`.
+ */
+export const mapStream: {
+    <A, E, R>(
+        f: (changes: Stream.Stream<NoInfer<A>, NoInfer<E>, NoInfer<R>>) => Stream.Stream<NoInfer<A>, NoInfer<E>, NoInfer<R>>,
+    ): (self: View<A, E, R>) => View<A, E, R>
+    <A, E, R>(
+        self: View<A, E, R>,
+        f: (changes: Stream.Stream<NoInfer<A>, NoInfer<E>, NoInfer<R>>) => Stream.Stream<NoInfer<A>, NoInfer<E>, NoInfer<R>>,
+    ): View<A, E, R>
+} = Function.dual(2, <A, E, R>(
+    self: View<A, E, R>,
+    f: (changes: Stream.Stream<NoInfer<A>, NoInfer<E>, NoInfer<R>>) => Stream.Stream<NoInfer<A>, NoInfer<E>, NoInfer<R>>,
+): View<A, E, R> => make({
+    get get() { return self.get },
+    get changes() { return f(self.changes) },
+}))
+
 /** Converts typed failures from both channels into `Result` values. */
 export const result = <A, E, R>(
     self: View<A, E, R>,
diff --git a/packages/effect-lens/src/Subscribable.ts b/packages/effect-lens/src/Subscribable.ts
index 9e6d107..53fc83c 100644
--- a/packages/effect-lens/src/Subscribable.ts
+++ b/packages/effect-lens/src/Subscribable.ts
@@ -40,6 +40,25 @@ export const mapOptionEffect: {
     onNone: () => Effect.succeed(Option.none()),
 })))
 
+/**
+ * Allows transforming only the `changes` stream of a `Subscribable`.
+ */
+export const mapStream: {
+    <A, E, R>(
+        f: (changes: Stream.Stream<NoInfer<A>, NoInfer<E>, NoInfer<R>>) => Stream.Stream<NoInfer<A>, NoInfer<E>, NoInfer<R>>,
+    ): (self: Subscribable.Subscribable<A, E, R>) => Subscribable.Subscribable<A, E, R>
+    <A, E, R>(
+        self: Subscribable.Subscribable<A, E, R>,
+        f: (changes: Stream.Stream<NoInfer<A>, NoInfer<E>, NoInfer<R>>) => Stream.Stream<NoInfer<A>, NoInfer<E>, NoInfer<R>>,
+    ): Subscribable.Subscribable<A, E, R>
+} = Function.dual(2, <A, E, R>(
+    self: Subscribable.Subscribable<A, E, R>,
+    f: (changes: Stream.Stream<NoInfer<A>, NoInfer<E>, NoInfer<R>>) => Stream.Stream<NoInfer<A>, NoInfer<E>, NoInfer<R>>,
+): Subscribable.Subscribable<A, E, R> => Subscribable.make({
+    get get() { return self.get },
+    get changes() { return f(self.changes) },
+}))
+
 /**
  * Combines the current values and streams of changes from multiple `Subscribable` values.
  */